1. Field of the Invention
The present invention relates to cathode ray tubes, and more particularly to a cathode ray tube in which coma aberration is reduced.
2. Description of the Prior Art
The applicant of the present invention has previously proposed a cathode ray tube as shown in FIG. 1 (Japanese Pat. Appln. No. 156167/83).
In FIG. 1, reference numeral 1 designates a glass bulb, numeral 2 a face plate, numeral 3 a target surface (photoelectric conversion surface), numeral 4 indium for cold sealing, numeral 5 a metal ring, and numeral 6 a signal taking metal electrode which passes through the face plate 2 and contacts with the target surface 3. A mesh electrode G.sub.6 is mounted on a mesh holder 7. The electrode G.sub.6 is connected to the metal ring 5 through the mesh holder 7 and the indium 4. Prescribed voltage, for example, +1200 V is applied to the mesh electrode G.sub.6 through the metal ring 5.
Further in FIG. 1, symbols K, G.sub.1 and G.sub.2 designate a cathode to constitute an electron gun, a first grid electrode and a second grid electrode, respectively. Numeral 8 designates a bead glass to fix these electrodes. Symbol LA designates a beam limitting aperture.
Symbols G.sub.3, G.sub.4 and G.sub.5 designate third, fourth and fifth grid electrodes, respectively. These electrodes G.sub.3 -G.sub.5 are made in process that metal such as chromium or aluminium is evaporated or plated on inner surface of the glass bulb 1 and then prescribed patterns are formed by cutting using a laser, photoetching or the like. These electrodes G.sub.3, G.sub.4 and G.sub.5 constitute the focusing electrode system, and the electrode G.sub.4 serves also for deflection.
A ceramic ring 11 with a conductive part 10 formed on its surface is sealed with frit 9 at an end of the glass bulb 1 and the electrode G.sub.5 is connected to the conductive part 10. The conductive part 10 is formed by sintering silver paste, for example. Prescribed voltage, for example, +500 V is applied to the electrode G.sub.5 through the ceramic ring 11.
The electrode G.sub.3 and G.sub.4 are formed as clearly seen in a development of FIG. 2. To simplify the drawing, a part which is not coated with metal is shown by black line in FIG. 2. That is, the electrode G.sub.4 is made so-called arrow pattern where four electrode portions H.sub.+, H.sub.-, V.sub.+ and V.sub.-, each insulated and zigzaged, arranged alternately. In this case, each electrode portion is formed to extend in angular range of 270.degree., for example. Leads (12 H.sub.+), (12 H.sub.-), (12 V.sub.+) and (12 V.sub.-) from the electrode portions H.sub.+, H.sub.-, V.sub.+ and V.sub.- are formed on the inner surface of the glass bulb 1 simultaneously to the formation of the electrodes G.sub.3 .about.G.sub.5 in similar manner. The leads (12 H.sub.+).about.(12 V.sub.-) are isolated from and formed across the electrode G.sub.3 and in parallel to the envelope axis. Wide contact parts CT are formed at top end portions of the leads (.sub.12 H.sub.+).about.(12 V.sub.-). In this case, each of the leads (12 H.sub.+).about.(12 V.sub.-) is made sufficiently narrow not to disturb the electric field within the electrode G.sub.3. For example, in an envelope of 2/3 inches (circumference of the electrode G.sub.3 =50.3 mm), width of each of the leads (12 H.sub.+).about.(12 V.sub.-) is made 0.6 mm. That is, the sum of each area of the four leads (12 H.sub.+).about.12(V.sub.-) is made only 4.8% of the total area of the portion of the electrode G.sub.3 which includes the leads (12 H.sub.+).about.(12 V.sub.-) (length d of lead x circumference). In FIG. 2, symbol SL designates a slit which is provided so that the electrode G.sub.3 is not heated when the electrodes G.sub.1 and G.sub.2 are heated by means of induction heating from outside of the envelope. Symbol MA designates a mark for angle in register with the face plate.
In FIG. 1, numeral 13 designates a contactor spring. One end of the contactor spring 13 is connected to a stem pin 14, and the other end thereof is contacted with the contact part CT of the above-mentioned leads (12 H.sub.+).about.(12 V.sub.-). The spring 13 and the stem pin 14 are provided for each of the leads (12 H.sub.+).about.(12 V.sub.-). The electrode portion H.sub.+ and H.sub.- to constitute the electrode G.sub.4 through the stem pins, the springs and the leads (12 H.sub.+), (12 H.sub.-) and (12 V.sub.+) and (12 V.sub.-) are supplied with prescribed voltage, for example, horizontal deflection voltage varying in symmetry with respect to 0 V. Also the electrode portions V.sub.+ and V.sub.- are supplied with prescribed voltage, for example, vertical deflection voltage varying in symmetry with respect to 0 V.
In FIG. 1, numeral 15 designates another contactor spring. One end of the contactor spring 15 is connected to a stem pin 16, and the other end thereof is contacted with the above-mentioned electrode G.sub.3. Prescribed voltage, for example, +500 V is applied to the electrode G.sub.3 through the stem pin 16 and the spring 15.
Referring to FIG. 3, equipotential surface of electrostatic lenses formed by the electrodes G.sub.3 .about.G.sub.6 is represented by broken line, and electron beam B.sub.m is focused by such formed electrostatic lenses. The landing error is corrected by the electrostatic lens formed between the electrodes G.sub.5 and G.sub.6. In FIG. 3, the potential represented by broken line excludes the deflection electric field E.
Deflection of the electron beam B.sub.m is effected by the deflection electric field E according to the electrode G.sub.4.
If distance from the beam limitting aperture LA to the target surface 3 (envelope length) is represented by l, length x of the deflection electrode G.sub.4 and distance y from the beam restricting aperture LA to the center of the electrode G.sub.4 are made with the following values, for example, so as to obtain good aberration characteristics. ##EQU1##
For example, in an envelope of 2/3 inches, l=46.6 mm, length of the electrode G.sub.3 (from the beam limitting aperture LA to the electrode G.sub.4)=9.3 mm, length of the electrode G.sub.4 =17.1 mm, length of the electrode G.sub.5 =18.2 mm, distance from the electrode G.sub.5 to the target=2 mm.
If the beam shape on the target surface 3 is observed in the image pickup tube shown in FIG. 1, a teardrop shape is seen as shown in FIG. 4A and in FIG. 4B where a circular shape is seen at the center but the current density distribution is deviated at the deflection to the right or to the left. In other words, so-called coma aberration is significantly produced in the image pickup tube shown in FIG. 1. If the coma aberration is significantly produced, the modulation degree is lowered at the right side of the frame and the uniform resolution is not obtained and the visual sense is insured. In addition, the amount of the coma aberration is represented by the distance between the original center 0 of the beam and the real position 0' of maximum density.